1. Field of the Invention
The subject invention relates to a switched-mode power supply which stabilizes an output voltage by sampling feedback information during a certain time interval.
2. Description of the Related Art
U.S. patent application Ser. No. 08/927,831, filed Sep. 11, 1997, now U.S. Pat. No. 5,831,839 (Attorney Docket No. PHN 16,282), discloses a switched-mode power supply which comprises a transformer with a primary winding and a secondary winding. The primary winding is connected to an input supply voltage via a switching device in order to obtain a periodically interrupted primary current through the primary winding by opening and closing the switching device under the control of a drive signal. The secondary winding is connected, via a rectifying diode, to a parallel arrangement of a smoothing capacitor and a load to supply a DC output voltage to the load. The transformer also includes an auxiliary winding which, due to coupling with the secondary winding, supplies an auxiliary voltage which is closely related to the DC output voltage of the secondary winding during a period of time when the rectifying diode conducts. A controller circuit is coupled to the auxiliary winding and receives feedback information for generating a drive signal for the switching device.
The controller circuit includes a sample-and-hold circuit for periodically sampling a current in the auxiliary winding, and for storing the sampled signal on a storage element. A drive circuit, coupled to the storage element, determines the drive signal for the switching device based on the sampled signal stored on the storage element. It is the object of the storage element to store an averaged sampled signal which is indicative of the DC output voltage. Due to the switching of the switching device, there are times when the auxiliary voltage drops below a regulation level. To prevent an erroneous level in the stored sampled signal, the sampling is only conducted when the current in the auxiliary winding is above a predetermined level which is equivalent to approximately 83% of the regulation level.
While this sampling control works adequately, Applicants have found that this type of control is subject to errors. In particular, the auxiliary voltage only remains at the intended level when there is current flowing in the secondary winding of the transformer. When this current drops to zero, the auxiliary voltage begins to ring. Any information sampled during the time that the auxiliary voltage is ringing is error information.